The present invention relates to a method for presoldering a contact for an electrical switching device with CuAg-based hard solder, over which the contact is connectable with a contact carrier for which purpose the hard solder must be applied as a flat layer, preferably with a defined contour, for example using a solder foil or the like, with the applied solder layer being melted. The term "contact" refers to an individual contact point, but can also refer to contact strips or sections to be used in cutting individual contact points to size. In addition the present invention relates to a semi-finished product for use as a contact in an electrical switching device, especially a contact point, contact strip, or contact section, with at least one contact layer and one solder layer being in largely intimate contact with the solder side of the contact over its entire area and having the structure of a melt.
Contact points manufactured by molding technology are usually attached by hard soldering to the contact carriers of switching devices. For this purpose, the solder is applied as a flat layer to the back of the contact point or the contact material in the form of a strip, for example using a solder foil. If the contact is initially in the form of a contact strip or section, from which contact points are later cut to length, a solder layer can also be produced initially by roll bonding. In particular, solder foils can be cut to length simply using an ultrasound technique to form a solder pad and attached uniformly to the solder side of contact points, as described in the German Patent Document No. DE-A-40 24 941. Contacts presoldered in this manner can be processed further mostly automatically and attached within the framework of an integrated manufacturing process, for example by inductive heating and soldering to the contact carrier.
In the latter energy-induced process, problems can occur because solder from the solder side rises above the narrow sides of the contact point and gets on the switching surface. Because of such uncontrollable processes, the switching behavior of the contacts can be affected in an undesirable manner during the proper use of the switching device.
It is therefore desirable for the solder to be applied in a smooth layer during presoldering of contacts, with the area of the solder layer generally being smaller than the area on the solder side of the contact. This is intended to prevent wetting of the narrow sides of the contact with solder.
Usually, especially for switching devices used in energy technology, contacts with a contact material layer with the silver metal composition (AgMe) or silver metal compound (AgMeV, where V is a compound), especially silver metal oxide (AgMeO) are used. Frequently such contacts have a two-layer structure, i.e. they consist of the contact layer itself and a layer of pure silver on the solder side. A solder suitable for this purpose is known which is based on copper and silver, which also can contain phosphorus in particular.
When, according to the prior art, solder foils are applied to the solder sides of contacts, it may be seen that when the solder melts, the solder preferably initially melts at the edges of the solder layer and air or gas inclusions between the solder foil and the contact point usually turn into a central bubble on the solder side of the contact point. To eliminate such bubbles which cause problems with subsequent processing of the presoldered contact points, as well as the associated irregularities on the surface of the solder layer, the solder must be heated until the bubble bursts as a result of the increase in its internal pressure, and breaks up. At such high temperatures, the solder generally runs onto the contact side of the contact point.
Patent Document No. GB-A-11 62 887 describes a method for continuous fastening of a narrow solder strip to a metal strip, for example a contact surface. The solder strip is briefly melted to form an intimate bond with the metal strip and has the structure of a melt. The shape of the solder strip is generally lost in the process, with at least a convex surface being obtained. U.S. Pat. No. 2,216,510 discloses a method for manufacturing contacts in which shaped parts are stamped from soldered parts and are further processed. The plates, presoldered using different methods, are then rolled to ensure a flat surface.